Fixed #35194 -- Fixed case-insensitive operations in GeneratedFields on Postgres 15.6+.

[sarahboyce]

Trac ticket number

ticket-35194

Branch description

Postgres >= 12.18, 13.14, 14.11, 15.6, 16.2 changed the way the immutability of generated and default expressions is detected in postgres/postgres@743ddaf.

This reverts the optimization of using UPPER instead of ILIKE for postgres case-insensitive comparisons as UPPER can return different value depending on the collation and thus is not immutable.

See: https://code.djangoproject.com/ticket/3575

I can see a StackOverflow discussion on how using LOWER/UPPER LIKE is faster than using ILIKE by roughly ~17%, however the advice is also "just use an index".

So....

• Do we think this is a viable solution to the problem?
• If yes, should I note/document the potential performance regression to case-insensitive un-index-ed Postgres queries in the release note?

I guess users could chose to use upper/lower themselves to achieve what we were doing in the background if they experience a performance regression and do not want to use an index 🤔

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.